As a sensor which optically measures a refractive index of a liquid or the like, there has been known a surface plasmon sensor. In the measurement of an incident angle characteristic (absorption curve) of reflectance ρ when an incident light having a wavelength λ is incident on a surface of metal, the reflectance ρ rapidly decreases at a certain angle of incidence (hereinafter referred to as absorption angle θsp). This phenomenon is called the plasmon resonance absorption, and is a phenomenon which is associated with electromagnetic wave coupling between an incident light and a surface plasmon excited on a surface of metal. As a result of the capture (resonance absorption) of electric power of the incident light into the surface of metal, reflected light intensity decreases.
The surface plasmon sensor is a sensor which measures a refractive index of a specimen such as a liquid by making use of the plasmon resonance absorption. With respect to the surface plasmon sensor, there have been known a surface plasmon sensor where a surface of a prism is coated with a thin metal film and a surface plasmon sensor having the periodic structure where grooves are formed on a surface of metal at equal intervals as disclosed in patent literature 1, for example.